A process is known for making calcium sulfate alpha-hemihydrate suitable for a construction material from a moist finely divided gypsum obtained by desulfurization of flue gas from a power plant fired by brown coal or lignite, especially from a gypsum obtained by flue gas desulfurization from a wet flue gas desulfurization unit. This process proceeds by recrystallization transformation of the calcium sulfate dihydrate contained in the gypsum in the presence of saturated steam.
Different processes are known for transforming calcium sulfate dihydrate into calcium sulfate alpha-hemihydrate. My invention is based on a process for making calcium sulfate alpha-hemihydrate from natural gypsum described in Ullmanns Encyklokadie der technischen Chemie, Bd. 12, 1976, Seite 301 (Ullmans Encyclopedia of Industrial Chemistry, 12, 301(1976)).
In this process, calcium sulfate dihydrate pieces, namely naturally occurring gypsum pieces, are fed to an autoclave and are converted to calcium sulfate alpha-hemihydrate pieces in the autoclave in the presence of saturated steam at a temperature of 130.degree. to 135.degree. C.
This alpha-hemihydrate product is dried above the thermal stability limit temperature of calcium sulfate dihydrate and is ground up for further use.
More specifically, the gypsum removed from a natural deposit is broken up into a grain size of 150 to 300 mm, is filled into baskets and is fed to an autoclave in the baskets.
The autoclave is directly or indirectly heated with steam from 130.degree. to 135.degree. C. The heating is controlled so that a pressure of 4 to 5 bar builds up in about four hours according to a saturated steam curve. Then the autoclave is emptied.
The alpha-hemihydrate gypsum is introduced into a drying chamber in the baskets and dried under standard pressure at about 105.degree. C. and subsequently fine ground.
In the surface regions of the pieces of material one finds well-defined calcium sulfate alpha-hemihydrate crystals which grow in a more or less needle shape.
In the core of the pieces of material after autoclaving structures with a diffuse crystal pattern and also residual calcium sulfate dihydrate are found even after very long treatment times. The crystal portion and also the surface fine structure are not controllable with the steps of this known process. Crystal portion means the grain size and structure of the surface of the crystal. The surface fine structure refers to the topography of the surfaces of the crystal. The quality needs improvement because of these reasons.
With so-called chemical gypsum, as formed in a finely divided state for example in phosphoric acid manufacture, it is known to mix the chemical gypsum with water to form a suspension or slurry and to deliver it to a flotation unit for removal of organic impurities.
Subsequently, both the water soluble and insoluble impurities are separated by a counterflow wash in a scrubber tower or in a hydrocyclone. Then the gypsum/water slurry is pumped continuously into an autoclave and transformed by recrystallization at a temperature of about 150.degree. C. and an appropriate saturated steam pressure into calcium sulfate alpha-hemihydrate.
Additives for control of the pH-value and for changing the crystal pattern can be metered into the autoclave and a product alpha-hemihydrate with various properties is obtainable.
In this known process, however, the expensive purification steps are troublesome and the large quantities of water required for crystallization lead to problems in drying and separation. Here distinct calcium sulfate alpha-hemihydrate crystals arise more by chance and control of the process in regard to crystal pattern and surface fine structure of the crystals formed is not provided. By contrast, manufacture of calcium sulfate alpha-hemihydrate with a special and well defined crystal pattern and also a well defined surface fine structure leads to a product with special properties for a variety of applications in the construction industry.